The proposed studies are concerned with the use of an animal model we have developed for the study of malformations induced by isotretinoin (13-cis-retinoic acid, Accutane); malformations which are comparable to those induced by this drug in humans. Since its introduction in September 1982, isotretinoin has become a widely used drug which is very effective for the treatment of severe cystic acne. As is the case with other retinoids (vitamin A), isotretinoin was known to be teratogenic in experimental animals. Despite warnings against use during pregnancy, there have been between 200 and 300 known exposures. In the prenancies which have been permitted to go to term, high incidences of craniofacial, heart and thymic abnormalities have occurred. Malformations in our animal model are very similar to those seen in human newborns and it has been possible to show that interference with neural crest development is a major feature in their pathogenesis. In addition, we have evidence that a metabolite, 4-oxo-isotretinoin (which reaches concentrations 3-5 times that of the parent compound in the serum of patients undergoing treatment) is also teratogenic. In this application, we propose to complete descriptive morphological studies using scanning EM, light microscopy and transmission EM. An immunohistological procedure will be utilized to assist in the identification of neural crest cells. Pharmacokinetic studies of isotretinoin and 4-oxo-isotretinoin which have been initiated will be completed. Regimens for maintenance in mice of blood levels of both compounds which are similar to human therapeutic levels are being developed. Following maternal administration, the levels of these compounds in the embryos will also be determined. Further studies concerning the mechanisms by which isotretinoin alters the development of neural crest and other cell populations in this model are also proposed. These include the use of 3H-isotretinoin and frozen section autoradiography to determine whether the compound is concentrated in crest cells at certain stages of their development as suggested by other studies using 14C-tretinoin (14C-all-transretinoic acid). Alterations in cell proliferation will be studied using 3H-thymidine autoradiography. Alterations in fibronectin and cell adhesion molecule will also be examined.